PROJECT SUMMARY/ABSTRACT A startling 40% of tuberculosis (TB) cases in endemic countries are ?missing? (~4.2 million cases globally remain undiagnosed or unreported)! Undiagnosed cases may substantially contribute to disease transmission as ~30% are smear positive while having an asymptomatic or minimally symptomatic presentation. These undiagnosed cases serve as a reservoir in the community and severely undermine TB control efforts. In the recently completed NIH-funded CASS I study we performed cough aerosol sampling on 453 patients with newly diagnosed TB (detected by passive case finding) and found ~30% were highly infectious ? containing culturable M. tuberculosis bacilli in their respirable cough aerosols (<10uM infectious inhalable particles that initiate disease). Multivariate analysis revealed relatively healthier, HIV negative patients who were not on TB treatment and had a high bacillary load were more likely to be highly infectious. We have also developed a novel low-cost mobile mini-clinic housing a novel TB diagnostic and HIV/CD4 testing capabilities. Using this mobile mini-clinic we have screened 4700 individuals in the communities of Cape Town and found 475 cases of suspected TB, 54 of which had confirmed TB. By combining these two techniques we found an unsettling ~17% of undiagnosed TB cases are highly infectious. Sizable gaps exist in our knowledge of community-based TB patient infectiousness and transmission. We do not know the ?bioprofile? of infectious undiagnosed cases in the community nor their degree of infectiousness, and a chasm remains in our understanding of patient level infectiousness and TB transmission. These have implications for targeting contact tracing investigations and the development of a scalable screening tool to measure infectiousness. In the proposed study we will employ active case finding using a novel mobile-mini clinic to detect ?missing? TB cases and establish their degree of infectiousness using cough aerosol sampling (Aim 1), determine a biosignature and bioprofile of infectious but minimally symptomatic undiagnosed individuals (Aim 2), and accurately model the impact of active case finding on disease transmission using estimates generated in the study, thus informing policy. In summary, this project will elucidate the bioprofile of undiagnosed minimally symptomatic active TB cases in the community ? ?the missing millions?, and further our understanding of the fundamental biology of TB transmission, potentially uncovering targets to interrupt disease spread.